It is becoming increasingly important to conserve energy and reduce power losses and power supplies with low power consumption are becoming more and more important. Such power supplies find applications in many situations, for example as standby power supplies in electrical devices (e.g. in televisions, washing machines), within external power supplies for supplying power to detect whether the electrical device is connected or not and to switch on the main power supply (e.g. within a portable telephone charger where the telephone is placed in a cradle for charging) or as standalone power supplies for electrical devices that require low power consumption, including low power external power supplies (e.g. a night light, plugged into the AC wall socket, to provide dim lighting).
In a first known arrangement, a power supply (used in a number of applications) comprises a transformer, the primary winding of which is connected directly to an AC power supply, the secondary winding of which provides an output voltage for the electrical device. To have low power consumption for this arrangement, the current through the primary winding of the transformer (which is connected directly to the AC supply) must be small. In order to have a small current, the impedance of the primary winding, as seen by the AC supply, must be large. With a typical AC supply frequency of 50 or 60 Hz, to have a large impedance in the primary winding, a large inductance will be required. To obtain such a large inductance in the primary winding, more turns are required, which will make the transformer impractically large. Alternatively, to avoid a large transformer, the wire used for the turns can be made thinner, but this means higher resistance which, in turn, means more losses. Essentially, to have a very low power consumption in this arrangement, we need a perfect inductor of high inductance and this is just not feasible.
A second known arrangement is known as a switching mode power supply (SMPS) and there are several different implementations of SMPS. Whilst SMPS provides advantages over the first arrangement, the rapid switching in that arrangement creates a large amount of noise. Also, SMPS are more complex in design and are more costly.
Currently, standby power supplies, such as those described above, typically have a power consumption of several hundred milliwatts or even as high as several watts. However, a typical power requirement of control circuits to “wake up” a device from standby can be as low as only a few milliwatts. So there is a big mismatch between the actual power required for a device in standby mode and the power consumed.